Typically, an axial flow turbine includes a rotor, a casing, and one or more turbine stages. Each stage has a stationary and rotational component. The rotational component is a wheel with a plurality of buckets, termed a bucket row, attached to its outer circumference. The wheel or wheels of each stage are co-axially mounted to a shaft which rotates about a first axis, also referred to as the rotational axis of the turbine. The assembly of the shaft and wheels is called the turbine rotor. The stationary flowpath component of the axial turbine stage is typically either a nozzle ring or a diaphragm. A plurality of nozzle passages are formed between nozzle vanes circumferentially arrayed about each stationary flowpath component. The nozzle passages are positioned radially about the first axis to line up with the buckets of the turbine rotor axially behind the nozzle passages.
Each of the constituent parts of the turbine discussed above have an upstream and a downstream side. The upstream side is the side of a part into which the motive fluid enters the object. Similarly, the downstream side is the side of a part where the motive fluid exits. In an axial flow turbine, the upstream and downstream sides are arrayed axially with respect to the rotational or first axis. As shown in FIG. 4C, typically an upstream edge of each nozzle vane is oriented to be approximately in line with the rotational or first axis and thus at an angle of about zero degrees with the rotational or first axis (the x-axis in FIG. 4C is the same as the rotational or first axis). In each stage, a stationary flowpath component is positioned upstream of a wheel assembly.
Typically, motive fluid such as steam, is repeatedly accelerated and directed by the nozzle passages between the nozzle vanes in such a way as to allow the adjacent downstream bucket row to extract energy from it. This energy is manifest as a torque on the rotating rotor and is ultimately available as shaft power from the turbine.
A turbine casing surrounds the turbine stage components. The stationary flowpath components which are diaphragms extend radially inward from the casing to the shaft.
The casing also provides an inlet and exhaust for the motive fluid. The inlet portion of the casing, called the inlet casing, is where the motive fluid initially comes into the turbine. This inlet casing typically has a series of circuitous passages which directs motive fluid to the first stage stationary component. Typically, the first stationary flowpath component is a nozzle ring, instead of a diaphragm. The nozzle ring is a ring shaped plate attached to the downstream face of the turbine inlet casing directly upstream of the first stage blade row.
For many turbine designs the motive fluid flow rate is controlled upstream of the turbine by a flow control device. The flow control device typically includes one or more control valves to control the flow rate and is connected upstream to a centralized source of motive fluid.
A problem with typical prior axial flow turbines is the configuration of the inlet casing. Usually, the motive fluid is directed from the control device to the inlet side of the nozzle ring and is presented to the inlet of the first stage nozzle passages in a purely axial orientation. Examples of such designs are shown in FIG. 5 of U.S. Pat. No. 4,592,699 to Maierbacher, which is herein incorporated by reference, and FIG. 1 of U.S. Pat. No. 4,840,537 to Silvestri, which is herein incorporated by reference. As illustrated in these patents, complex shapes for the inlet casing passages are required to orient the motive fluid in a purely axial orientation. The extra bends or curves in these passages increase the frictional losses in the motive fluid. Thus, less energy is available in the motive fluid for extraction by the turbine.
In some designs this efficiency loss is overcome by increasing the passage cross section areas to lower the average fluid velocities. Unfortunately, this solution requires larger passages which in turn leads to greater wall thickness, so that the bulk and cost of the inlet casing, and therefore the turbine, becomes unfavorable. Additionally, lowering average fluid velocities may not be enough. Even if the turbine has lower average fluid velocities, it may still have regions of locally high velocity, for example tight turns, where frictional losses in the motive fluid are still high.
Another shortcoming related to these prior designs is with the direction of flow upstream of the nozzle ring. Since the motive fluid is flowing in an axial orientation, the nozzle vanes have to both accelerate the fluid and also change its direction from axial to almost tangential (or circumferential) to efficiently convert the energy in the motive fluid in the bucket row following the nozzle vanes. Typically, the nozzle vanes have an inlet angle of about zero degrees with respect to the first or rotational axis and need to turn the motive fluid about seventy-five degrees for proper orientation for transfer to the subsequent row of buckets because of the generally axial nature of the flow of motive fluid from stage to stage inherent in an axial flow turbines. The energy loss in the motive fluid from this turning in the nozzle passage is even larger than in the inlet casing passages because the velocity of the motive fluid in the nozzle passages is much higher.
To reduce the amount of turning required, some turbine designs present the motive fluid to the first stage in a tangential direction with respect to the first or rotational axis, rather than axial direction, such as shown in U.S. Pat. No. 3,861,821 to Keller et al. which is herein incorporated by reference. To a large extent the unnecessary turning in the nozzle passages is eliminated by this design, but a significant swirl chamber between the downstream side of the nozzle passages and the row of buckets is introduced. The swirl chamber can introduce radial non-uniformity in the flow of motive fluid at the upstream side or inlet to the buckets which reduces overall efficiency of the turbine. Further, the additional distance the motive fluid travels in the swirl chamber is relatively long and thus increases the frictional losses in the motive fluid.
Another approach to solving this problem is shown in U.S. Pat. No. 5,215,436 to Puzyrewski, which is herein incorporated by reference. Again, motive fluid flow is directed tangentially into the turbine, but in this case the turbine has no nozzle passages. The absence of nozzle vanes gives less control over motive fluid flow distribution and uniformity, thus introducing more losses in the energy extraction process and reducing overall efficiency of the turbine.